Hi, Does any one know the details of the shifting of focus in macro/closeup as a lens is stopped down? I've heard that the xenars do this ... you have to refocus at stopped down aperture.This is a pain because the image is so dim at f22-32 etc....any solutions? Does the focus shift move closer or further from the film plane when stopped down? Thanks, Emile.

-- Emile de Leon (knightpeople@msn.com), October 08, 2001

Answers

Hi, I've read what Ansel Adams wrote about that, he mentions the Dagor and similar lens designs. But never heard about that shifting as shooting with Tessar-types nor made any such experiences with that lenses.

-- Thomas Vaehrmann (TVaehrmann@web.de), October 08, 2001.

Hi Emile

I have a 50 years old Schneider Super Angulon 90mm and on thad one I have a focus shift closer to the camera, but only in the 2 meterers and closer area. I would do it like so: Take the lens sharp open who the sharpness should be, take the halfe way down to f 11 overcorrect it a bit and close the rest down further. Or buy a very cheap lens like the Scheider G-Glaron which is corrected for 1:1 and it solved your problem! Good light!

-- Armin Seeholzer (armin.seeholzer@smile.ch), October 08, 2001.

As far as I know, the main lenses that suffer from focus shift are Dagors, dagor-related lenses and Imagons. These lenses all have significant spherical aberration, intentionally in the case of the Imagon. The Dagor, Angulon and Super Angulon are discussed in Rudolf Kingslake's book A History of the Photographic Lens. Kingslake's diagrams show the Angulon (patent 1930) to be what he calls a reversed Dagor. The Super Angulon (patent 1957) is an unrelated design.

My suggestion for testing whether your lens has focus shift in closeups: focus on the filament of a clear-glass lightbulb, perhaps using a dimmer. Then stop down, turn up the brightness, and recheck the focus. If it is a problem, in the studio you can try using extra illumination for focusing. In the field, sometimes placing a flashlight as a focus aid is possible.

-- Michael Briggs (MichaelBriggs@earthlink.net), October 09, 2001.

Thanks much for the info....Good Light! Emile.

-- Emile de Leon (knightpeople@msn.com), October 11, 2001.